<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            display: flex;
            flex-direction: row;
            /* justify-content: center;  居中对齐*/
            /* justify-content: space-between;两边贴平再平分 */
            /* justify-content: space-around;平分剩余空间 */
            /* align-items: center; 沿侧走居中 单行使用*/
            /* align-items: stretch; 沿车轴拉伸 单行使用*/
            
            width: 800px;
            height: 500px;
            background-color: black;
        }
        span {
            width: 200px;
            /* height: 100px; */
            background-color: red;
            margin-top: 10px;
            margin-left: 10px;
            /* flex: 1; */
        }
    </style>
</head>
<body>
    <div>
        <span></span>
        <span></span>
        <span></span>
    </div>
</body>
</html>